Apparatus and method for evaluating the surface insulation resistance of electronic assembly manufacture

ABSTRACT

A test vehicle for an SIR testing system is described. A circuit board having a plurality of interleaved circuit patterns is provided with replica components mounted thereon. The interleaved circuit patterns have different conductor spacings to permit a qualitative evaluation of electronic assembly manufacturing processes. The replica components have connection leads which are soldered to circuit pads on the circuit board. The circuit pads are connected into two groups which are used along with the interleaved circuit patterns to evaluate the manufacturing process using SIR measurements.

BACKGROUND OF THE INVENTION

The present invention relates to a technique for measuring the qualityof an electronic assembly soldering process. Specifically, a testvehicle is provided which may be used in a surface insulation resistance(SIR) test to evaluate the quality of an electronic assembly solderingprocess.

The manufacture of electronic circuits includes mounting integratedcircuit components onto a single printed circuit board of a module whichmay be used in a larger electronic system. The circuit board includesconductor patterns which terminate in a connection to either acomponent, or a connector for making external connections to the circuitboard. The process of preparing the board is well known, which caninvolve a variety of soldering and cleaning techniques. The solderprocess typically involves the application of flux, solder, and heat tothe solderable areas on the circuit board in order to form a solderedconnection to components, followed by cleaning to remove unwantedsoldering byproducts. The solderable areas include not only connectionpads for components, but plated through via holes which extend to thesurface of the printed circuit board. The via holes provide a connectionbetween conductors on opposite sides of the printed circuit board.

The soldering techniques typically employed to make external connectionsto the circuit board include solder paste reflow, wave, fountain,thermode, laser, and hot gas soldering. Subsequent removal ofpost-soldering byproducts is usually accomplished using aqueous orsolvent-based chemistries in a cleaning machine, or without cleaning ifno-clean soldering materials are used.

The process of removing the soldering byproducts becomes more difficultas the density of circuit conductors on the circuit board increases, andas the profile or dimensions of the components on the board and therelevant spacing between the components and the board decreases. Anincomplete removal of the soldering byproducts has been known to producelatent electrochemical migration or corrosion effects between conductiveelements on the assembly. This is particularly acute when a mildlyactivated resin or organic acid based flux systems are used. Theelectrochemical migration or corrosion effects may result in latentproduct reliability problems, such as open circuits caused by corrosionor electrical shorts resulting from electrochemical migration.

One of the techniques which has been developed for evaluating theability of an electronic assembly process to adequately remove solderingbyproducts is known as the surface insulation resistance (SIR) testingtechnique. The SIR testing methodology has been developed with industrystandard specifications controlled by the Electronic IndustryAssociation (EIA) and the Institute for Interconnecting and PackagingElectronic Circuits (IPC). SIR testing employs the use of a specialprinted circuit board test vehicle which is processed through anelectronic assembly manufacturing process. The test vehicle includesfour wiring patterns of interleaved conductors which are not normallyconnected together. The wiring patterns comprise two electricalconductors having a width of 0.016 inches and a spacing of 0.020 inchesbetween the conductors. The test vehicle is subjected to the solderingand cleaning process without any external components or a solder maskwhich is used in the majority of useable electronic assemblies. Thus, insome ways the standard SIR testing techniques are deficient since theprocess of removing soldering byproducts from a useable circuit board iseffected by the component placement on the printed circuit board.

As new component technology reduces the profile of the various circuitpackages which are attached to a printed circuit board, the spacingbetween the printed circuit board and the components gets smaller,presenting a challenge for removing soldering byproducts which mayreside between components and circuit board. Whereas previous componentbodies are spaced 0.01 inches from the board surface, facilitating theremoval of soldering byproducts which enter the space, the newercomponents provide a spacing of 0.002 inches which increases thedifficulty in removing soldering byproducts which may be trapped beneaththe component. This in turn increases the risk for latentelectrochemical migration or corrosion which cannot be tested using atest vehicle which does not account for the reduced spacing between thecircuit board and components.

The additional improvements in reducing the size of electricalconductors also presents a greater difficulty in evaluating thepotential for electrochemical migration or corrosion. Whereas theprevious test vehicles used a conductor width of approximately 0.016inches and conductor spacings of 0.020 inches, newer printed circuitdesigns allow for a conductor spacing of 0.006 inches versus theprevious 0.020 inch conductor spacing.

Given the foregoing improvements in electronic assembly density, abetter test vehicle is needed for evaluating the soldering and cleaningprocesses in this new high density environment.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a SIR test vehicle forevaluating printed circuit manufacturing processes for electronicassemblies.

This and other objects of the invention are provided for by a printedcircuit board which supports a plurality of interleaved circuitpatterns. The interleaved circuit patterns have a pair of spaced apartconductors which terminate on a via hole on the circuit board. The viaholes on the circuit board are connected by a conductor within the holeto a conductor on the underside of the circuit board. A replicacomponent having the dimensions and lead configurations of an actualcomponent is supported on the circuit board over at least one of theinterleaved patterns. The test vehicle with the replica component issubjected to the manufacturing process through which actual circuitboard assemblies are to be fabricated. The interleaved circuit patternsprovide an ability to measure the effects of electrochemical migrationor corrosion which occurs as a result of unwanted soldering byproductsbeing incompletely removed from the circuit board surface. The replicacomponents are supported above the circuit board at distances which arecontemplated for actual components which are to be used on circuitboards. The effects of soldering byproducts which remain on theelectronic assembly, and particularly soldering byproducts which residebetween the replica component and the circuit board, may be determinedby measuring the resistance between the interleaved conductors of thecircuit pattern.

In a preferred embodiment of the invention, not only is the resistancebetween the interleaved circuit patterns measured as an indicator of themanufacturing cleaning process effectiveness, but the replica circuitcomponent connections to the circuit board are divided into two groups,and resistance measurements are made between groups of connections as anindication of the potential for electrochemical migration or corrosioneffects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a test vehicle for making SIR measurementsbefore component placement;

FIG. 2 is a side view of FIG. 1 showing component placement on thecircuit as board;

FIG. 3 is a top view of one quadrant of the circuit board constitutingthe test vehicle; and

FIG. 4 is a bottom view of the circuit board quadrant of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a plan view of a test vehicle circuit board 10is illustrated. The circuit board 10 is divided into four quadrants 6,7, 8 and 9 having identical interleaved circuit conductor patterns andconnector configurations. The circuit board of FIG. 1 is intended toinclude four surface mounted replica components 11 (shown only in FIG.2) which approximate the actual component configuration which will be onprinted circuit boards being assembled in a tested soldering andcleaning process. It is contemplated that the test vehicle 10 of FIGS. 1and 2 will support replicas of state of the art integrated circuitpackages, such as a TQFP package having a height of 0.055 inches. Thesepackages include between 32 and 256 connection leads about thecircumference of the package. The minimum spacing of the leads isapproximately 0.005 inches. The footprint offered by the foregoing TQFPpackage is approximately 0.079 inches, and when mounted on a quadrant 6,7, 8 and 9 of the circuit board 10, substantially covers fourinterleaved circuit patterns 14, 15, 16 and 17. The leads 19 around thecircumference of the package connect to four sets of circuit board pads21, 22, 23 and 24 placed around the circumference of circuit patterns 14through 17. In the embodiments shown, 128 connection leads are connectedto circuit board solder pads of sets 21 through 24.

Each of the quadrants 6 through 9 of the printed circuit board 10supports an identical replica component 11 having the same number ofleads supported on the same number of circuit pads. Each of theindividual quadrants 6 through 9 provide for separate tests of surfaceinsulation resistance once the test vehicle comprising circuit board 10and mounted replica components 11 have been put through an electronicassembly manufacturing process.

The individual quadrants 6 through 9 of the circuit board 10 have aplurality of via holes 32 which form terminations for a pair ofconductors of each of the interleaved circuit patterns 14 through 17.Each of the ends of the conductors for the circuit patterns 14 through17 are connected to a via hole 36 through 43, where they are connectedto conductors running on the other side of the circuit board 10 toterminals of connector 26 as will be evident from the discussion of FIG.4. The significance of the location of the set of via holes 32 is tosimulate a circuit board hole pattern which results in flux used in thesoldering process entering the space 35 between the replica components11 and circuit board 10. Space 35 containing the circuit patterns 14through 17 represent a challenge for the manufacturing process toeffectively remove flux residue or soldering byproducts which may havemigrated into the space 35 through the via holes 32 as well as laterallyaround the component leads 19. The space 35 may be as small as 0.002inches using the TQFP circuit packages.

FIG. 3 illustrates on a larger scale an individual quadrant 6 of thetest vehicle of FIG. 1. Each of the circuit patterns 14 through 17 is aninterleaved pattern of two conductors, and the spacing between theinterleaved patterns is different for each pattern. By havingdifferently spaced interleaved patterns on the circuit board, it ispossible to quantify the manufacturing process performance as it relatesto electrochemical migration or corrosion.

The conductors 14a, 14b, 15a, 15b, 16a, 16b, 17a and 17b simulate thecircuit conductor of the printed circuit boards used in state of the artmanufacturing processes. The spacing, between conductors of eachinterleaved circuit pattern may be, for example, 0.006 inches, 0.012inches, 0.019 inches, and 0.032 inches.

Each of the circuit pads sets 21 through 24 comprise individual circuitpads which are grouped into two groups 21(a), 21(b), 22(a), 22(b),23(a), 23(b) and 24(a) and 24(b). Group (a) and (b) are connected toalternate circuit pads, and to individual terminals of the connector 26.For instance group 21(a) connects to terminal 50 and group 21(b)connects to terminal 51 through the via hole 49 and conductor 44. Groups22(a) and 22(b) are connected to terminals 52 and 53, groups 23(a) and23(b) are connected to terminals 54, 55 and groups 24(a) and 24(b) areconnected to terminals 57, 58.

Similar connections are provided for circuit pad sets 22, 23 and 24,with each of these sets of circuit pads divided into two groups, andeach group connected to a respective terminal of the connector 26. Anytype of connector may be substituted for the connectors 26 shown. Forexample, a series of printed conductive pads at the edge of the boardmay be used so the board can fit into a slot.

FIG. 4 shows how the via holes 36 through 42 and 48 and 49 are connectedto respective terminals of connector 26 at the edge of the board. Acentral via hole 56 is left unconnected. This via hole presents a worstcase flux or soldering byproduct residue N2 removal scenario since itresides under the center of replica component 11.

The test vehicle in accordance with the preferred embodiment is runthrough an actual manufacturing process for soldering the components 11to the boards, including a fluxing operation, a soldering operation, anda subsequent cleaning operation to remove soldering byproducts from theelectronic assembly. The quality of each of these processes is reflectedin the measured resistance between each of the conductors of circuitpatterns 14 through 17, as well as the resistance between two groups ofeach of the sets of component pads 21, 22 and 23. Following themanufacturing processes, these resistance values are measured at theconnector 26, and an evaluation of the manufacturing process is made.For instance, soldering byproducts which have migrated underneath thereplica components 11 which were not removed during cleaning, mayproduce corrosion and/or electrochemical migration, affecting theresistance measurements of circuit patterns 14 through 17. The fourquadrants 6 through 9 of the test vehicle provide the capability ofindependent SIR measurements. The combination of replica components 11,circuit board conductor patterns 14 through 17, and their relativeposition with respect to the via holes provide a rigorous evaluation ofthe soldering and cleaning process for increased circuit densities.

The foregoing description of the invention illustrates and describes thepresent invention. Additionally, the disclosure shows and describes onlythe preferred embodiments of the invention, but as aforementioned, it isto be understood that the invention is capable of use in various othercombinations, modifications, and environments and is capable of changesor modifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings, and/or the skill orknowledge of the relevant art. The embodiments described hereinabove arefurther intended to explain best modes known of practicing the inventionand to enable others skilled in the art to utilize the invention insuch, or other, embodiments and with the various modifications requiredby the particular applications or uses of the invention. Accordingly,the description is not intended to limit the invention to the formdisclosed herein. Also, it is intended that the appended claims beconstrued to include alternative embodiments.

What is claimed is:
 1. A test vehicle for an electronic assemblymanufacturing process comprising:a printed circuit board supporting oneor more test patterns, each test pattern including at least oneinterleaved circuit pattern, said interleaved circuit pattern having afirst and second spaced apart conductors having ends which connect to aconnector on said circuit board; and a replica component supported onsaid circuit board over said one or more test patterns, said interleavedcircuit pattern providing an electrical resistance measured through saidconnector which varies in accordance with the quality of a manufacturingprocess for said test vehicle.
 2. The test vehicle according to claim 1wherein a plurality of said interleaved circuit patterns is provided onsaid circuit board having different spacings between said spaced apartconductors.
 3. The test vehicle according to claim 2 wherein each ofsaid interleaved circuit patterns have first and second ends connectedto individual via holes in said circuit board.
 4. The test vehicleaccording to claim 1 wherein said replica component includes a pluralityof leads which are connected to a plurality of circuit pads on saidcircuit board.
 5. The test vehicle according to claim 4 wherein saidcircuit pads are divided into two groups, each pad of a group beingconnected in common, the resistance between each of said groups being anindication of the quality of said manufacturing process.
 6. The testvehicle according to claim 5 wherein said two groups comprise alternateof said circuit pads.
 7. A test vehicle for an electronic assemblymanufacturing process comprising:a circuit board having a plurality ofinterleaved circuit patterns, each of said circuit patterns includingfirst and second conductors which are spaced apart and have first andsecond ends which are connected through at least one via hole to aconnector on said circuit board; a replica component having leads forconnection to a plurality of circuit pads on said circuit board duringsaid manufacturing process, said replica components covering at least aportion of said circuit patterns and at least one via hole, whereby aresistance between said first and second conductors indicates theeffects of said manufacturing process on the circuit patterns formedunder said replica component.
 8. The test vehicle according to claim 7wherein said circuit patterns each have a different spacing between saidfirst and second conductors.
 9. The test vehicle according to claim 7wherein said first and second conductors first and second ends terminateon a different via hole.
 10. The test vehicle according to claim 7wherein said circuit pads are connected in two groups so that theresistance between each of said groups may be measured as an indicationof said effects of said manufacturing process.